1. Field of the Invention
This invention relates to a method for the manufacture of printed circuit (wiring) boards and, in particular, to printed circuits boards made using a photosensitive dielectric material having enhanced copper to dielectric adhesion, wherein the dielectric material is used to form layers on the printed circuit board as a base for electrical circuitry and for photoforming vias.
2. Description of Related Art
Printed circuit boards are an integral part of electronic equipment and there have been continuing efforts to increase the interconnection density and electrical efficiency of printed circuit boards and thus decrease their size and cost. The technology relating to printed circuit boards goes back a number of years and, in general, a printed circuit is patterned on the surface of a dielectric material such as a plastic substrate using many techniques including electroless and electrolytic plating. These boards vary in design and may have a copper layer on each surface of the plastic substrate usually epoxy (termed 2-sided boards) or they can be multi-layer boards which have a plurality of interleaved parallel planar copper and epoxy layers. In both types, through-holes are drilled in the board and metal plated to facility connection between the copper layer circuits.
The trend in recent years to decrease the size of electrical equipment has resulted in the need for electronic packaging which satisfy a variety of requirements including a high density of electrical circuitry, high performance, and reliability and low cost. To achieve these goals, the printed circuit board was redesigned to reduce the through hole size since this was found to be a limiting factor in the manufacture of printed circuit boards. The presently used printed circuit boards still use a plastic substrate such as a FR4 glass epoxy material which contains surface circuitry but, instead of utilizing multiple layers of such an epoxy substrate to build a multi-layer printed circuit board, additional layers are built-up using thin dielectric layers made of a permanent photosensitive material on which the circuitry is plated. Signal interconnection between wiring planes is done by a photo via hole process in the dielectric layers by which via holes can be made much smaller and more economically than by conventional mechanical (e.g., drilling) or other methods. High density wiring is thus achieved by the use of small photo via holes, fine circuit lines made by thin copper conductors and a thin dielectric layer. The photosensitive material is also known as a photo definable dielectric and the terms may be interchanged.
To produce such high density printed circuit boards, a single-sided board, two-sided board or a multi-layer board may be used as the starting substrate. A photosensitive dielectric material such as an epoxy is applied to the substrate by any of a number of processes such as a curtain coating process in which the liquid dielectric flows like a curtain from a narrow slit with the substrate board running under the slit and which board receives a coating of the dielectric material. Typically, signal via holes are then formed by photoetching of the epoxy after an initial cure. The etched epoxy layer is then finally cured to produce a dielectric material having the necessary characteristics of hardness, low dielectric constant, high glass transition temperature, low moisture absorption and low cure temperature, among others. A circuit conductive layer is then plated on the dielectric layer by any of the known techniques such as the subtractive, additive, and semi additive methods as is well known in the art.
For purposes of illustration, plating of the dielectric layer will be described for the subtractive process (termed panel plating) on a circuit patterned on a two-sided printed circuit board containing through-holes although the invention is applicable to other methods of manufacture. The photosensitive dielectric is applied to the board and allowed to tack dry. The board is exposed to radiation to form the desired via holes and developed to expose the holes. The board is then cured to harden the dielectric. Before electroless plating, the surface of the dielectric is preferably etched using an etchant such as an alkaline permanganate solution to increase the adhesion of copper plating to the dielectric surface. A copper layer is then bonded to the textured dielectric surface using a high press lamination process or preferably a wet process comprising an electroless copper flash followed by electroplating with copper to the desired thickness. A photoresist is then coated onto the copper layer and is exposed and developed so that the areas of the copper that are to ultimately form the desired copper pattern remain covered. The portions of the copper layer that are not covered by the delineated photoresist are removed by etching leaving the desired copper pattern. After etching, the photoresist is stripped leaving the desired circuit pattern on surface of the dielectric material. Additional layers are formed by repeating the above steps of applying the dielectric material, forming via holes, developing, curing, texturing, copper plating, applying a photoresist, imaging, developing, etching and stripping to form a multilayer board. When the last layer is formed, it is generally desirable to apply a solder mask to protect the circuitry on the surface of the board. In another fabrication process termed a pattern plating process, the above steps are followed up to the electroless copper flash step. After the copper flash step, a resist is applied, exposed and developed. The board is then electroplated, the remaining resist stripped and the copper flash coating etched. The above steps are then repeated to form a multilayer board and a solder mask applied to the upper layer.
Other fabrication processes include direct metallization processes wherein a plastic substrate may be electrolytically plated without the need for any prior electroless plating as described in U.S. Pat. Nos. 5,358,602 and 5,376,248 both of which patents are hereby incorporated by reference.
A number of patents have issued directed to making high-density printed wiring boards (PWB's) using such a thin photosensitive dielectric material and fabrication process. Exemplary are U.S. Pat. Nos. 4,795,693; 4,902,610; 4,927,983; 5,097,593; 5,246,817; 5,266,446 and 5,344,488. An article entitled "Surface Laminar Circuit Packaging" by Tsukada et al. published in 1992 IEEE, pages 22-27 also shows this method. The above patents and article are hereby incorporated by reference.
An important consideration in the above processes regardless of the fabrication process used is the adhesion of the copper plating to the dielectric surface. Without proper adhesion, the reliability of the circuitry will be compromised and defective printed circuit boards will result. The problems in plating plastic dielectric substrates such as epoxy or other such materials are well known in the art and a number of methods have been developed over the years to improve the adhesion of the metal plating to the substrate. These methods generally employ oxidants to etch the surface of the plastic prior to plating and include chromic acid, sulfuric acid and alkaline permanganate solutions. The toxicity of the chromium compounds and their potential hazards as water pollutants and the safety precautions needed with sulfuric acid have increased the commercial use of permanganate solutions, particularly alkaline permanganate solutions and a number of patents have been granted in this area. For example, U.S. Pat. No. 3,252,351 shows the etching of acrylonitrile-butadiene-styrene interpolymer (ABS plastics). U.S. Pat. Nos. 4,042,729 and 4,054,693 disclose stable, highly active etching solutions containing particular ratios of manganate ions and permanganate ions by controlling the pH in the range of 11-13. U.S. Pat. No. 4,425,380 is specifically directed to cleaning through-holes of residual manganese prior to plating. U.S. Pat. Nos. 4,592,852 and 4,629,636 assigned to the assignee of the present invention disclose improved alkaline permanganate compositions for etching printed circuit boards by incorporating a secondary oxidant in the solution capable of oxidizing formed manganate ions to permanganate ions. The disclosures of the above patents are hereby incorporated by reference.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a method for making a printed circuit board including a multi-layer printed circuit board made using a photodefinable dielectric which dielectric has enhanced copper to dielectric adhesion.
It is another object of the present invention to provide an improved printed circuit board including a multi-layer printed circuit board made using a photodefinable dielectric having enhanced metal (e.g., copper) to dielectric adhesion.
A further object of the invention is to provide a permanent dielectric polymer material having enhanced metal (e.g., copper) to dielectric substrate adhesion which is suitable for use in making printed circuit boards and which material may be applied to the boards using a variety of coating techniques such as curtain coating.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.